In my aforementioned copending application Ser. No. 627,410, hereinafter referred to as my copending application, it is observed that prior art devices for harmonic drive or strain wave gearing have received one or both of those names as a result of operation wherein areas of mating relationship or engagement between the teeth of two ring gears are peripherally propagated in the form of a sinusoidal or substantially sinusoidal wave representative of a wave deflection or strain wave in one of the gears. The concept of harmonic drive is set forth in some detail in U.S. Pat. No. 2,906,143 to Musser, issued Sept. 29, 1959. Basically, the Musser invention constitutes a motion transmitting device in which is provided a rigid circular ring gear, a flexible ring gear of different diameter from but coaxial with the rigid ring gear, and some form of strain inducing device by which the flexible ring gear is driven and at the same time deflected to force its teeth into meshing relationship with the teeth of the rigid ring gear at a plurality of circumferentially spaced points separated by areas of noncontact therebetween. In this manner, rotational driving of the flexible ring gear by the strain inducer results in the propagation of a strain wave about the periphery of the flexible ring gear, accompanied by relative rotation between the two gears.
According to the Musser patent referred to above, the circular gear or ring gear is an annular ring provided with internal teeth, i.e., teeth projecting radially along the inner periphery. The flexible gear or strain gear is also annular, being disposed within the rigid ring gear, fabricated of a thin resilient material capable of elastic deflection, and provided with external teeth projecting radially about and from the outer periphery thereof. The pitch diameters of the two gears differ as a consequence of the difference in number of teeth between the gears, the strain gear having fewer teeth than the ring gear by a number equal to or a multiple of the number of positions of mating engagement between the gears, in accordance with the predetermined distortion of the strain gear by the strain inducer when the gears are disposed one within the other. By virtue of this arrangement, at any given instant of time a large percentage of the teeth of the two cooperating gears are in contact, more than 50 percent of each.
As generally disclosed by Musser, the strain inducer is mounted on a shaft with which the strain gear and ring gear are coaxial, and has a configuration adapted to exert forces on the inner periphery of the strain gear, when inserted into a position inside the latter, so as to deflect or distort the wall of the strain gear to produce the desired configuration of mating relationship between the two gears at a plurality of circumferentially spaced positions. Thus, as the strain inducer undergoes rotation the strain gear is driven such that the teeth of the two gears enjoy complete engagement at only a limited portion of each position of mating relation and have varying degrees of engagement at either side of each limited portion, being completely separated from one another in areas approximately midway between positions of mating relation in those cases where sufficient disparity exists between pitch diameters (and tooth differential) of the gears. Accordingly, a strain wave is propagated about the periphery of the strain gear, one complete revolution of which is characterized by a tooth movement equal to the tooth differential between the gears, the gears undergoing relative rotation. Musser emphasizes that the strain inducer need not be the driving element; rather that any of the three elements (i.e., ring gear, strain gear or strain inducer) may be the driving element and either of the remaining two the driven element. The gear having the largest number of teeth per radian moves in the same direction as the strain inducer when the latter is the driving element.
Among the variations of strain inducer or wave generator mentioned by Musser in his aforementioned patent are a pair of electromagnetic embodiments, one involving polyphase energization and the other single-phase energization. More recent patents of related disclosure indicate a recognition that electromagnetically energized strain wave gearing or harmonic drives were not actually previously constructed, and proceed to disclose suitable forms thereof. One of these patents, U.S. Pat. No. 3,169,201, entitled "Electromagnetic Harmonic Device" issued Feb. 9, 1965, in the names of Spring et al., contemplates elimination of the mechanical strain inducer or wave generator cam and its bearing, and of the shaft coupled thereto, as provided in the invention disclosed in the aforementioned Musser patent, and utilization in its stead of an electromagnetic drive system including a stator and a rotor. The rotor of the Spring et al. invention comprises a plurality of thin flat magnetically permeable plates of substantial nonretentivity projecting radially from and lying in planes intersecting the common axis of the output shaft and the gears, toward respective lineal positions adjacent the surface of the strain gear (also termed "flexspline") remote from the ring gear (also termed "circular spline"), and arranged to pivot against the surface when subjected to magnetizing force. The stator comprises an even number of evenly spaced pairs of solenoid coils (with magnetic cores) disposed in a circular array coaxial with the array of rotor plates and spaced from portions of the plates protruding from the flexspline. Progressive radial distortion or deflection of the flexspline to produce a mating relation between the two splines (gears) at a plurality of points is effected by energization of an appropriate plurality of the coils in a desired sequence, thereby sequentially forcing the magnetic plates (i.e., deforming the armature) against the internal surface of the flexspline, resulting in strain wave meshing of the splines as in the aforementioned Musser patent. The Spring et al. patent discloses this electromagnetic wave generator as an actuator for a digital stepping motor, wherein diametrically opposed pairs of solenoid coils are energized in sequence by a control circuit, also disclosed, to produce radial deflection of the flexspline into an elliptoidal shape, with progressive circumferential strain wave deflection in discrete steps.
In U.S. Pat. No. 3,169,202, issued Feb. 9, 1965 in the names of Proctor et al. still other types of electromagnetic actuators for strain wave gearing or harmonic drives are disclosed, these actuators having a continuously rotating field and differing one from another primarily in respect to type of armature. The basic configuration by which the strain wave deflection is propagated is, however, entirely similar to that disclosed in the aforementioned Musser and Spring et al. patents. The armatures described in the Proctor et al. patent include an endless chain of magnetically permeable rigid links, adjacent links pivotal relative to one another, the chain disposed adjacent the surface of the flexspline remote from the ring gear; a laminated core, the laminations being in successive plates along the axis of the actuator and coaxial therewith, the periphery of the successive laminations being interrupted by equiangularly spaced slots in which magnetic powder is disposed, adjacent the internal surface of the flexspline; and a coiled magnetically permeable flat strip positioned adjacent the internal surface of the flexspline. In the case of each of these types of armature the continuously rotating field produced by appropriate energization of an associated stator is effective to distort the respective armature, thereupon subjecting the flexspline to deflecting forces.
In my copending application, I disclose an improvement upon the aforementioned prior art forms of harmonic drive system, wherein the basic concept of the wobble plate type of electromagnetic motor is employed together with sequential switching of the stator windings to produce the desired stepped rotation of a wobble plate or wobble disk rotor. Accordingly to an embodiment of that invention, a pair of cooperating circular ring gears of the same diameter are provided with teeth projecting from confronting planes. At least one of the ring gears is rigid, fastened to the internal surface of the larger diameter wall of a concentric double cylindrical walled housing having a bridge joining the walls at a common end thereof to form a "doughnut cup" shaped enclosure. This enclosure or housing contains a laminated annular magnetic core having a plurality of equiangularly spaced coils, corresponding to the desired number of motor phases, wound thereon. A shaft extends within the inner wall of the housing along the axis thereof and is mounted for rotation in bearings retained at either end of the space encompassed by the inner wall. An armature or rotor in the form of a magnetically permeable circular plate is retained on the shaft for relative rotation therewith and has adjacent its periphery along a planar surface of the plate the second of the aforementioned ring gears, one ring gear (preferably that on the armature) having at least one less tooth than the other ring gear, the teeth of the two gears normally spaced from one another. In the preferred embodiment the armature is rigid, as is its ring gear. One end of the magnetic core in the housing confronts the armature and as the phases (field windings) of the motor are energized in the desired switching format, the armature is successively pulled toward each energized coil. Accordingly, the teeth of the two ring gears are forced into mating engagement, i.e., intermesh, at only one limited region of each gear at any given instant of time. As the coil switching progresses the armature wobbles about the shaft, the position at which its ring gear meshes with the stationary ring gear fastened to the housing propagating sinusoidally along that gear in accordance with the wobble motion. This constitutes a substantially sinusoidal wave motion, the armature constituting a mass rotating at an extremely low rotatory rate which depends upon tooth differential, number of motor phases, and switching format for the phases. A relative rotation occurs between the two ring gears, and if the armature has the fewer teeth it rotates that number of fewer teeth for each revolution of the wobble (i.e., each revolution of the intermeshed position of the gears), and in a reverse direction to the direction of rotation of the wobble. A second pair of ring gears is provided by which the armature is coupled to the shaft to drive the latter in accordance with armature rotation so that the shaft undergoes discrete (stepped) rotational motion in accordance with the switching format phase energization of the motor. This second pair of gears operates to transmit torque to the shaft in a positive and reliable manner, yet with a minimum of frictional drag or loss of power due to wear. In the harmonic drive system of my copending application, it will be noted that the teeth of the cooperating ring gears are engaged at only one portion of the overall ring, so that complete or partial meshing occurs between only a small number of teeth at any given instant of time. Consequently, the load or force on the shaft is not truly balanced, a condition which is somewhat disadvantageous when compared with harmonic drive systems of the prior art.
However, the invention disclosed in my aforementioned application enjoys several advantages over prior art harmonic drives, resulting in a decided overall improvement. For example, no power need be applied to the "strain inducer" unless actual stepped rotation is desired. In contrast, the prior art systems require application of power to the strain inducer in order to deform the flexspline even when the system is in a condition undergoing no rotational movement but in a state of preparedness to do so. Over a lengthly period of continuous use this can result in a substantial power saving in favor of the apparatus of my previous invention.
Moreover, the prior art flexsplines have been found to be resonant at a number of different frequencies, a condition which results in loss of smooth and efficient performance. This cannot readily occur in the drive system of my earlier invention, because unlike the prior art strain gears, there is no thin elastic tubular flexspline structure.
Wobble plate motors per se are, of course, known insofar as basic concepts are concerned. The invention disclosed in my copending application, however, combines the broad wobble plate or wobble gear concept with two pairs of mating gears, one gear of each pair disposed on a wobble plate of magnetically permeable material, and the other gear of one pair fixed by attachment to the housing, while the other gear of the other pair is coupled in driving relation to the shaft; along with a driving or energizing circuit by which the motor windings are sequentially switched to produce the stepped rotation with rotor "wobble around."
In only one prior art electromechanical wobble gear actuator of which I am aware is there an arrangement utilizing at least two pairs of gear teeth, one row of teeth of each pair on the wobble plate and the other row of teeth of each pair affixed to a frame member and to a shaft, respectively. In that prior art actuator, however, the wobble gear, i.e., the rotor, has a web portion formed of a high coercive force permanent magnet material radially magnetized with a center of one polarity and a periphery of opposite polarity. Stator poles are disposed about the periphery of the frame with windings thereon, which when energized, drive the polarized armature. A major problem resides in the maintenance of perfect mating contact between the rigid gear surfaces. This ordinarily requires the use of precise construction measures by which all gear teeth are cut to provide complete meshing between each pair of associated gears. It will be appreciated that such a requirement is accompanied by high cost of production and is essentially ruled out from a practical standpoint. A further requirement is the establishment of sufficient force on gears to insure complete and continuous meshing of the teeth as the wobble plate rotates. Although the desired mating contact may be assured in a new set of gears by the aforementioned exacting and expensive methods of cutting the teeth, nevertheless gear teeth wear rather rapidly when subjected to continuous or lengthly periods of operation. A related problem, then, is the maintenance of desired mating contact between teeth of associated gears despite wear, and more particularly, uneveness of teeth in the initial gear sets. With conventional wobble plate coupling arrangements, imperfections in associated gear teeth and wear of the teeth as operation continues are accompanied by play, i.e., excess and usually nonuniform freedom, between the gears themselves, which is transmitted also to members to which the gears are fastened, such as input or output shaft. This situation creates nonuniformly of rotation of the gears, with intervals of speedup and slowdown during each period of rotation, compounding the uneveness of wear.
The use of special biasing arrangements has been suggested, for example the employment of springs arranged to force the gears and gear teeth together. However, if the spring force is excessive a substantial amount of energy must be expended to overcome that force and to cause the gears to mate, thereby reducing efficiency; and if insufficient spring force is available the gears will intermittently separate under conditions of loading.